Such a double flywheel is described in the specification of United Kingdom published patent application GB 2 160 296A. In that document, the torsion damper comprises a disc which extends radially inwardly so as to offer an engagement surface to the anti-friction bearing means. Fastening screws secure the first mass to the crankshaft of the engine.
In practice the screws pass through passages formed in the central hub. As a result, it is not possible to create a module that includes both the double flywheel and its associated clutch. In general terms, it is possible to tighten the screws by gaining access to them through openings formed in the diaphragm of the clutch and through the clutch friction disc or clutch plate; however, the heads of the screws may still come out and escape before the module has been fitted on to the crankshaft.
As is described in the specification of French published patent application FR 2, 554 891A, it is possible to locate the securing screws radially outwards of the anti-friction bearing means, with the second hub then being formed with through openings for passage of the heads of the screws and of a suitable screwdriving tool. In that document, the second hub is surrounded by the central hub. The second hub can also be so designed that it serves as one of the components of the double flywheel, so as to retain the heads of the screws. It is then simplest to provide a shoulder in one of the components of the double flywheel for retaining the heads of the screws. However, this solution is not entirely satisfactory, because if one of the screws breaks during the assembly operation, the whole assembly has to be dismantled.